JPS6116908B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION When natural gas is in short supply, and the price of natural gas is high because of the scarcity, natural gas in major gas producing countries must be Substantial efforts are being made to produce liquefied natural gas from hydrocarbon feedstocks near gas wells. The production of liquefied natural gas (LNG) requires significant amounts of energy, requiring distillation to remove condensable components from the hydrocarbon feedstock and a cooling cycle to liquefy the methane obtained from this distillation cycle. The entire cycle must be extremely efficient to be competitive with other fuel sources. Furthermore, as the energy cost for producing liquefied natural gas increases, profits will decline accordingly.

US Pat. No. 3,702,541 discloses a method for removing condensable components from a hydrocarbon feedstock containing a substantial proportion of methane. In that method, the feedstock is cooled to a temperature below -18°C and expanded in a turbine to produce a mixture of gas and condensate, which is fractionated to remove condensable components and remove methane. An overhead fraction is obtained containing: The overhead fraction is cooled and re-expanded in a heat exchanger, the expanded gas then flows through a heat exchanger, and then the expanded gas is re-expanded using the energy output of the expansion device. Compressed. This gas is released into the pipeline.

No. 3,792,590 discloses passing a natural gas feedstock through a series of dryers and filters to dry it, dividing the feedstock into multiple fractions and a minor fraction; A process for liquefying natural gas is disclosed which comprises passing a fraction of the fraction through a carbon dioxide absorbent and then through a cooling cycle for liquefaction. A large proportion of the fraction is mechanically expanded and passed through a heat exchanger for cooling to high temperature levels. The energy of expansion of the major fraction is used to drive the main compressor for the refrigeration cycle.

The method disclosed in U.S. Pat. No. 3,763,658 is improved by isenthalpically expanding a hydrocarbon feed gas from a feed pressure to a lower pressure and distilling the hydrocarbon feed at that lower pressure to produce a methane-enriched gas. It is also known to produce liquefied methane by obtaining an overhead fraction and then liquefying this overhead fraction in a cooling cycle.

In particular, the present invention provides a liquefaction method for liquefying a natural gas feed stream, which method comprises: (a) supplying the natural gas feed stream at an absolute pressure of 60.5 Kg/cm ² or greater; (c) cooling the feed stream by indirect heat exchange in a number of heat exchange zones with a refrigerant in a separate closed loop cooling system; (c) converting the feed stream into an overhead fraction formed in a subsequent step of fractionation (d); and the bottoms stream isentropically expanded to a first pressure below the critical pressure of both;
and thereby obtaining mechanical energy; (d) fractionating said expanded feed stream in a scrub column to produce a methane-enriched overhead fraction and a heavy hydrocarbon-enriched bottoms fraction; (e) cooling and partially condensing the partially condensed overhead fraction by indirect heat exchange with a multicomponent refrigerant in a separate closed-loop cooling system; and (f) (g) feeding said liquid fraction as reflux to said scrub column; (h) said methane-enriched vapor stream; is fed directly to a compressor at a temperature below -73°C, and the mechanical energy recovered in expansion step (c) above is used to compress the vapor stream to an absolute pressure of at least 47.8 Kg/cm ² into methane. (i) supplying the methane-enriched high-pressure stream to a heat exchange zone; and (j) supplying the methane-enriched high-pressure stream to the same polypropylene stream used in cooling step (e) above; cooling by indirect heat exchange in said cooling zone with a component refrigerant, liquefied and cooled below its liquefaction temperature; and (k) converting said liquefied and cooled methane stream below its liquefaction temperature into a liquefied natural gas product. It is characterized by extracting it as a flow.

The advantages of the present invention are not achievable with the prior art and include: (a) without substantial sacrifice of liquefaction efficiency;
Valuable gaseous components can be fractionated from hydrocarbon feedstocks at pressures well below the critical pressure, (b) in a relatively simple manner, i.e. before liquefaction;
Energy can be recovered from the hydrocarbon feedstock to increase liquefaction efficiency by using the energy gained in expanding the hydrocarbon feed to fractionation pressure to recompress the methane-rich overhead fraction. , and (c) recover methane from hydrocarbon feedstocks and liquefy the methane in a more efficient manner than previously used methods.

The invention will be explained with reference to the accompanying drawings. The accompanying drawing is a process flow sheet of one preferred embodiment of a cycle designed to produce liquefied methane from gaseous hydrocarbon feedstocks.

Generally, feedstocks suitable for use in the process of this invention have a methane content of about 60 to about 90 mole percent, with the balance being nitrogen and heavy hydrocarbons. These feedstocks are typically fractionated to produce a methane-rich overhead fraction and a heavy hydrocarbon-rich bottoms fraction.

The hydrocarbon feed enters the system through line 10 and passes through first heat exchanger 12 after carbon dioxide impurities have been removed. This heat exchanger 12
is the first of three cascade heat exchangers, which include C ₂ , C ₃
Or a single component refrigerant such as _C4 paraffin hydrocarbon is supplied.

Typically, propane is used as the single component hydrocarbon refrigerant because it has been found to achieve optimal temperatures at the most ideal pressures.

The hydrocarbon feedstock is cooled with the propane in heat exchanger 12 to a first temperature level of about 20° C. and passed through phase separator 14 from which condensed water is removed through line 16. ,
be discharged. This partially dried hydrocarbon feedstock is then passed through line 18 to either of a pair of dryers 20, each dryer 20 removing substantially all residual water from the hydrocarbon feedstock.
Each dryer contains a desiccant, such as activated alumina, and as is well known in the art.
It is configured so that it can be played alternately. The dried hydrocarbon feedstock is then passed through line 22 to a second single component refrigerant heat exchanger 24 where the feedstock is cooled to approximately -13C. This cooled hydrocarbon feedstock passes through line 25 to heat exchanger 2
4 and sent to a third single component heat exchanger 26. There, it is cooled to a temperature of approximately -33°C.

Typically, the feed stream exiting heat exchanger 26 is -
It flows at a rate of 26,201 moles of hydrocarbon feedstock per hour at a temperature of 33°C and a pressure of 60.5 Kg/ ^cm2 , and the hydrocarbon feedstock contains approximately 0.35% nitrogen, 83% methane, 10.5% methane in mole percent. Ethane, 3.7% Propane, 0.65%
of isobutane, 1.03% butane, 0.23% isopentane, 0.18% pentane and 0.18% hexane. (All pressures herein are absolute pressures.) The cooled feedstock is transferred to line 27
through a phase separator 29 where it is separated into a liquid phase and a vapor phase. This vapor phase is taken off through line 31 and then into the common shaft 3
Expander 33 and compressor 35 provided in 7
and an expander-compressor combination. This hydrocarbon feedstock is isentropically expanded from a pressure of about 60.5 Kg/cm ² to a pressure of 33.7 Kg/cm ² and during the expansion (as a result of the work expended) a temperature of about −58° C. cooled to temperature. The effluent from expander 33 is sent through line 39 and is combined with the liquid from line 41 coming from the bottom of phase separator 29, the latter having been previously depressurized through throttle valve 23. This combined stream is sent by line 43 to scrub column 28 from where heavy hydrocarbons are delivered to discharge line 30.
The gaseous hydrocarbons are separated from the gaseous hydrocarbons as condensate. Small amounts of lighter hydrocarbons, including methane, ethane, and propane, may also be removed and sent to a fractionation system (not shown) to provide replenishment refrigerant as described below. scrub tower 2
Most of the stream coming from the bottom of the scrub column 8 is recycled to the steam reboiler 32 to provide steam through the bottom tray of this scrub column. Typically,
This method requires removal of heavier hydrocarbons. This is because if the heavy hydrocarbons are not removed, they will freeze in the downstream liquefaction cycle and block the unit. Additionally, the content of components such as ethane, propane, butane, and gasoline in the feedstock, and the relative value of those components as separate products, may dictate the recovery of those components from the feedstock. often desired.

Fractionation at a feed pressure of 60.5 Kg/cm ² is undesirable. This is because the pressure of the desired methane-rich overhead fraction is substantially the same as or slightly higher than its critical pressure.

At and above the critical pressure, and even at pressures slightly below the critical pressure, the densities of the liquid and vapor are approximately the same, making it difficult to separate the gas from the liquid. Generally, the pressure is determined by the critical force of either the overhead or bottom fraction (whichever pressure is lower).
By reducing the pressure to at least about 20% less ^than Distillation or fractionation can be accomplished. Preferably, the pressure is reduced to a range of about 14.1 to about 45.7 Kg/cm ² so that optimal fractionation is achieved.

About 27.484 moles per hour of a methane-enriched (eg, about 93% methane) natural gas feed stream exits scrub column 28 as overhead vapor. Generally, the overhead fraction contains about 80% or more and preferably about 90% or more methane. This overhead vapor then passes through line 34 to the first
tube bundle 36 and is cooled by a multicomponent refrigerant from a temperature of about -66°C to a temperature of about -79.5°C. This cooled steam is removed from the first tube bundle 36 of the main heat exchanger 50 by line 36a and sent to the second phase separator 38, which phase separator 3
Further condensed hydrocarbons are separated from 8. This liquid condensate is returned through line 40 to scrub column 28 via pump 42 and line 44 to become the reflux of scrub column 28. The methane-rich overhead fraction leaves the top of phase separator 38 as vapor via line 45. The top fraction is transferred to phase separator 3.
The pressure and temperature when leaving 8 are approximately 32.7 Kg/cm ² and -80°C.

It is in the combination of the following steps that the plant efficiency is increased compared to the prior art. In this combined process, approximately 22,479 moles of overhead fraction per hour are removed from phase separator 38 through line 45 and sent by expander 33 to compressor 35 driven via shaft 37 and expanded The total work obtained from the vessel 33 compresses it to the highest possible pressure, that is, 47.8 Kg/cm ² . In other words, the energy to compress the methane-rich overhead fraction is obtained by utilizing the expansion energy of the hydrocarbon feed as it is reduced in pressure to the fractionation or distillation pressure. Until now, the methane-rich overhead fraction was sent to the liquefaction cycle without being compressed, resulting in a substantial reduction in the efficiency of the overall liquefaction cycle. Traditionally, the inlet pressure is often kept high throughout the distillation cycle to minimize power consumption, e.g.
It was customary to maintain a weight of 49.2 Kg/cm ² . However, this separation was inefficient. An advantage of the present invention is that the pressure can be reduced well below the critical pressure of either the overhead or bottom fraction to be produced, and that the desired fractionation substantially reduces the overall plant efficiency. This can be achieved without sacrifice. This is because the methane-rich overhead fraction can be compressed to a substantially higher pressure due to the energy gained during expansion, making the process more efficient.

After compression, the methane-rich overhead fraction can be liquefied by a conventional refrigeration cycle, ie, Joel-Thompson, isentropic expansion, or cooled by a liquefied refrigerant. The preferred liquefaction cycle, as shown in the drawings, uses a multicomponent refrigerant in a commercially available coiled heat exchanger.

In the liquefaction cycle, the compressed overhead fraction stream passes through line 47 to two-zone main heat exchanger 50.
to one tube circuit 48. This overhead fraction is sent upwardly through tube circuit 48 and cooled by a countercurrent of a first multicomponent refrigerant sprayed downwardly into the tube circuit from spray header 52. This multicomponent refrigerant generally consists of 2-12 mole percent nitrogen, 35-45 mole percent methane, 32-42 mole percent ethane, and 9-19 mole percent propane. In the example shown, the refrigerant is 10 mol%
of _N2 , 40 mol% _CH4 , 35 mol% _{C2H6 and}
It is composed _of 15 mol% _C3H8 . The methane-enriched overhead fraction is passed directly to a second tube circuit 54 and sent upwardly through this tube circuit and into a second direction where it is sprayed downwardly from a spray header 56. It is cooled by a multicomponent refrigerant fraction of the stream. This overhead fraction is withdrawn from the top of tube circuit 54 as a fully liquid, cooled below liquefaction temperature stream having a temperature on the order of -164 DEG C. and a pressure on the order of 38.7 Kg/ ^cm2 . This liquefied feed stream, which has been cooled significantly below its liquefaction temperature, is then expanded in valve 58 to a pressure on the order of 5.3 kg/cm ² and a temperature on the order of -161°C. Since it has been cooled well below its liquefaction temperature, no volatilization occurs and the liquid is discharged directly into a storage tank where it can be stored at atmospheric pressure and temperatures as low as -161°C.

Approximately 22,478 moles of LNG are obtained per hour.

Returning to heat exchangers 12, 24 and 26, propane or other single component refrigerant is
step 60 and a second step 60 including a suction flow in the intermediate wheel.
and a compressor having a step 62. The compressed propane is cooled and fully condensed in water cooler 64 and then in heat exchanger 12.
It is expanded in valve 66 from a pressure of 14.2 Kg/cm ² to a pressure of approximately 8.2 Kg/cm ² before entering, and is cooled from 40.6°C to 18.4°C by such expansion. Heat exchanger 12 as well as other propane heat exchangers may be of conventional design, such as having U-tubes immersed in liquid propane. However,
Some of this liquid propane evaporates during cooling of the hydrocarbon feed in the U-tube, and this vapor is transferred to line 6.
8 and returns to the intermediate wheel in step 62. The remaining liquid refrigerant is sent from heat exchanger 12 through line 70 to branch lines 72 and 90. The portion of propane in branch line 72 is expanded by valve 74 to a pressure on the order of 3.0 Kg/cm ² and transferred to heat exchanger 24 at a temperature on the order of -16°C.
will be introduced in A second portion of liquid refrigerant is evaporated in cooling the feed stream in heat exchanger 24 and returned to the first wheel of stage 62 through line 76. The remaining liquid propane is transferred to the heat exchanger 24.
through line 78 and valve 8
It is expanded to a pressure of about 1.3 kg/cm ² in 0 and introduced into the heat exchanger 26 at a temperature of about -37°C.
This portion of refrigerant evaporates during cooling of the feedstock, and this cooling vapor passes through lines 82 and 84 to stage 6.
It is returned to the suction side of 0. Thus, the feedstock is continuously cooled in three single-component refrigerant heat exchangers, where the same refrigerant is passed through a three-stage, cascading refrigerant cycle with progressively lower pressure and temperature. It is obvious that it is being used.

In addition to cooling the hydrocarbon feedstock in the cascade cycle described above, the single component refrigerant described above is utilized substantially to liquefy the overhead fraction in the main heat exchanger 50 and cool it below its liquefaction temperature. It can also be used for cooling and partial condensation of multi-component refrigerants. This cooling of the multi-component refrigerant by the single-component refrigerant is provided from the heat exchanger 12 through the main line 70 and the branch line 90. A second portion of liquid propane is used in heat exchangers 86 and 88. This portion of the propane refrigerant is expanded to a pressure of about 3.0 kg/cm ² by a valve 92, and introduced into the heat exchanger 86 at a temperature of about -16°C. A portion of the propane evaporates during cooling of the multicomponent refrigerant and is withdrawn from heat exchanger 86 through line 87 and step 62
is returned to the first wheel. The remaining liquid propane is sent from the heat exchanger 86 via the line 93 and the expansion valve 94 to the heat exchanger 88, where the propane is heat exchanged at a pressure of about 1.3 kg/cm ² and a temperature of approximately -37°C. Enter vessel 88. This portion evaporates during cooling of the feedstock, and the refrigerant vapor is returned to the suction side of compressor 60 via lines 96 and 84. A replenishment line 97 can be provided downstream of valve 66 to replace refrigerant lost from the propane cycle due to propane leaking into the atmosphere, so that adequate liquid propane is lacking in the system. Time Compressor 60
Fresh liquid propane can be added to the suction side of and 62.

10 mol% _N2 , 40 mol% _CH4 , 35 mol%
A multicomponent refrigerant consisting of C ₂ H ₆ and 15 mole percent C ₃ H ₈ is compressed in compressors 100 and 102 having an intercooler 104 and a postcooler 106.
The pressure of the multicomponent refrigerant vapor in line 108 may generally vary between about 35.2 and about 84.4 kg/ ^cm2 . In the illustrated example, the pressure in line 108 is approximately
It is 45.7Kg/cm ² and its temperature is about 41.1℃.
This multi-component refrigerant is passed through line 108 to heat exchanger 86 where it is cooled below its liquefaction temperature by propane and then passed directly into a second propane heat exchanger 88 from which 44.6 kg / ^cm2
It is released at a pressure of about -33°C. This multicomponent refrigerant is withdrawn through line 109 and sent to phase separator 110.

The liquid condensate in phase separator 110 is transferred to line 112
is sent to the tube circuit 114 of the main heat exchanger 50, where it is cooled to a temperature of about -112°C, which is lower than the liquefaction temperature. The liquid cooled below the liquefaction temperature is expanded to a pressure of about 3.5 Kg/cm by valve 116, whereby a small amount evaporates into vapor and its temperature drops to about -119°C. This liquid and its volatilized vapor are transferred to the pipe circuits 36, 48,
line 118 and spray header 5 to provide downwardly flowing refrigerant onto 122 and 114.
2 into the heat exchanger 50.

Returning to phase separator 110, the overhead vapor is passed through line 120 to tube circuit 122 where it is cooled and condensed by a downwardly sprayed polymorphic refrigerant. Tube circuit 122
The condensed multi-component refrigerant therein is sent directly to the second tube circuit 124 where it is cooled to a temperature on the order of -163 DEG C., which is below the liquefaction temperature. The liquid refrigerant cooled below its liquefaction temperature is expanded in valve 128 to a pressure of about 3.6 kg/cm ² , whereby a small portion of it is volatilized and vaporized, and its temperature is approximately -167°C.
descend to This liquid and volatilized vapor is injected into heat exchanger 50 via line 130 and spray header 56, and then into tube circuit 5.
4 and 124, which then joins with the multicomponent refrigerant from spray header 52. This combined multicomponent refrigerant is then evaporated in heat exchange with tube circuits 36, 48, 114 and 122. As a result, all this multicomponent refrigerant is recombined as a vapor phase at the bottom of heat exchanger 50, withdrawn, and passed through lines 136 and 1.
38 to the suction side of the compressor 100. As such, the multicomponent refrigerants in this system form separate closed cycles that most efficiently transport the methane overhead fraction from propane levels to a final cooling temperature of approximately -164°C below the liquefaction temperature. cooled down.

A replenishment line 140 and valve 142 may be provided so that such multi-component refrigerant can be added when needed to compensate for unavoidable refrigerant losses. As previously mentioned, this make-up refrigerant can be obtained by fractionating the hydrocarbons discharged from the scrub column through line 30 and adding additional nitrogen.

The embodiments of the invention described above are capable of optimally and desirably fractionating gaseous hydrocarbons from heavier hydrocarbons without substantially sacrificing liquefaction efficiency. In the prior art, for example, fractionation is approximately 49.2
It was carried out at a pressure of Kg/cm ² and then liquefied at the same pressure. Lower pressures were not used for fractionation because the power costs required to achieve the desired liquefaction efficiency would be correspondingly higher.

Other embodiments of the invention also achieve the overall effects of the invention. That is, fractionation can be performed at an optimal or desired pressure, and excellent liquefaction efficiency can be obtained. For example, if the hydrocarbon feed pressure is approximately 84.4 Kg/cm ² , it is expanded to 33.7 Kg/cm ² , fractionated, and then the energy obtained from the expansion is used to generate the overhead fraction. It can be recompressed to approximately 52.7Kg/cm ² and then liquefied. In this way, the desired fractionation can be achieved with good liquefaction efficiency. On the other hand, if the prior art method were used with the same pressure feedstock, the feedstock would expand isenthalpically, fractionate at 49.2 Kg/cm ² and then liquefy at 49.2 Kg/cm ² . It is clear that fractionation and liquefaction as used in the prior art utilizing 84.4 Kg/cm ² of feedstock is not as efficient as in the process of the present invention.

[Brief explanation of the drawing]

The figure is a process flow sheet of a preferred embodiment of a cycle designed to produce liquefied methane from gaseous hydrocarbon feedstocks. In the figure, 12, 24, 26, 50, 86, 88...
... Heat exchanger, 14, 29, 38, 110 ... Phase separator, 20 ... Dryer, 28 ... Scrub column, 3
3... Expander, 35, 60, 62, 100, 10
2... Compressor.

Claims (1)

Claims: 1. (a) providing a natural gas feed stream at an absolute pressure of 60.5 Kg/cm ² or more; (b) converting the feed stream to multiple heat sources by a refrigerant in a separate closed-loop cooling system; (c) cooling the feed stream by indirect heat exchange in an exchange zone; (d) fractionating the expanded feed stream in a scrub column to produce a methane-rich overhead fraction and a heavy hydrocarbon-rich overhead fraction; (e) cooling and partially condensing the overhead fraction by indirect heat exchange with a multicomponent refrigerant in a separate closed-loop cooling system; (g) feeding the liquid fraction as reflux to the scrub column, (h ) feeding said methane-rich vapor stream directly to a compressor at a temperature below -73° C. and using the mechanical energy recovered in said expansion step (c) said vapor stream to at least 47.8 Kg/cm ^{2 .} (i) supplying the methane-rich high-pressure stream to a heat exchange zone; and (j) compressing the methane-rich high-pressure stream to an absolute pressure of Cooling process (e)
cooling and liquefying by indirect heat exchange in the cooling zone with the same multicomponent refrigerant used in
and (k) extracting the liquefied and cooled methane stream below its liquefaction temperature as a liquefied natural gas product stream. Method. 2. A liquefaction process according to claim 1, characterized in that the natural gas feed stream is supplied at an absolute pressure in the range of 60.5 to 84.4 kg/cm ² . 3.47.8 methane-rich vapor stream during compression step (h)
A liquefaction method according to claim 1 or 2, characterized in that the liquefaction method is compressed to an absolute pressure in the range of ~52.7 Kg/cm ² . 4. The liquefaction method according to claim 1, 2 or 3, characterized in that the raw material stream is expanded to an absolute pressure within the range of 14.1 to 45.7 kg/cm ² in the expansion step (c). .